Abstract

The Interim Staff Guidance on burnup credit for pressurized water reactor (PWR) spent nuclear fuel (SNF), issued by the United States Nuclear Regulatory Commission's (U.S. NRC) Spent Fuel Project Office, recommends the use of analyses that provide an "adequate representation of the physics" and notes particular concern with the "need to consider the more reactive actinide compositions of fuels burned with fixed absorbers or with control rods fully or partly inserted." In the absence of readily available information on the extent of control rod (CR) usage in U.S. PWRs and the subsequent reactivity effect of CR exposure on discharged SNF, NRC staff has indicated a need for greater understanding in these areas. In response, this report presents a parametric study of the effect of CR exposure on the reactivity of discharged SNF for various CR designs, including Axial Power Shaping Rods, fuel enrichments, and exposure conditions (i.e., burnup and axial insertion). The study is performed in two parts. In the first part, two-dimensional assembly calculations are performed, effectively assuming full axial CR insertion. These calculations bound the effect of CR exposure and facilitate comparisons of the various CR designs. In the second part, three-dimensional calculations are performed to quantify the reactivity effect of CR exposure in a burnup credit cask environment and determine the effect of partly inserted CRs. The reactivity effect as a function of axial insertion depth is shown for the various CR designs considered. The results from the study demonstrate that the reactivity effect increases with increasing CR exposure (e.g., burnup) and decreasing initial fuel enrichment (for a fixed burnup). Further, CR exposure is shown to have a larger effect on discharge reactivity when it occurs later in the assembly burnup. For variations in CR design, there exists a direct relationship between the reactivity worth of the CRs and their effect on discharge reactivity -higher reactivity worth CRs result in larger effects on discharge reactivity. The effects are quantified and typical operating conditions are reviewed, enabling an increased understanding of the effect of CR exposure on the reactivity of discharged SNF. The report concludes with a discussion of the issues for consideration and preliminary recommendations to address the effect of CR exposure in burnup credit criticality safety analyses.